In infrared spectrometers, a beam of radiation from a source is directed through a series of mirrors or other focusing elements onto a sample held in a holder. The radiation transmitted through or reflected from the sample is then collected and directed onto a detector. In a dispersive instrument, the polychromatic IR beam from the source first goes through a monochromator. The monochromatic IR beam then goes through the sample and reaches the detector. The wavelength of the monochromatic IR beam changes with time. The sample absorbs IR radiation of different wavelength to a different extent. The detector measures the intensity of the IR radiation as a function of wavelength. A spectrum is constructed from the data of intensity vs. wavelength or wave number. In FTIR spectroscopy, the output signal from the detector is analyzed with known computer processing techniques to derive information concerning the structure and composition of the sample. Spectrophotometers measure the relative intensities of light in different parts of a spectrum.
The primary advantages of FTIR spectrophotometers are that entire wavelength ranges can be analyzed faster, with more energy throughput and with reduced background stray light. Such advantages are well known in the spectrophotometer art and, consequently, have given rise to the increasing use of FTIR spectrophotometers.
Conventionally, spectrophotometers can be of either single beam or double beam variety. In a single beam instrument, the sample and a blank, which consists of, for example, a sample holder or a sample cell filled with a solvent, must be measured separately. The spectrum of the blank (reference) is then subtracted from the spectrum of the sample. It is often necessary to disrupt the control environment in order to switch from sample to blank measurement. Consequently, the blank measurement is neither simultaneous with the sample measurement nor is it performed under identical conditions.
As a means of avoiding this inaccuracy, double beam systems were developed and are well known in the art. In such systems, one beam is directed through the sample cell or sample material whereas a second beam, following a substantially identical optical path--except for that portion of the path passing through the sample--is introduced. Normally, this is accomplished by positioning, both before and after the sample cell, rotatable or otherwise movable mirrors, or other beam direction control optical elements. Accordingly, the incoming or incident beam directed to the sample cell is intermittently redirected along a path similar in length and other optical characteristics to that of the sample beam but which path does not contain a sample. Thus, by optically or electrically subtracting the signal representative of the incident beam having passed through the sample from the beam having passed through the reference path, a more accurate measurement is achieved. Even in a double beam system, noise created by conventional sample holders is a problem.
Many of the FTIR spectrometer systems in commercial and laboratory use today make use of a Michelson interferometer to create a time varying light wave to pass through a sample of material. Variations in the light intensity due to interference in the sample chamber are created by a moving mirror in the interferometer.
In a basic Michelson-type interferometer, the radiation flux emanating from a source of monochromatic light, either in the visible or the invisible regions of the electromagnetic spectrum, impinges on a beam splitter after passing through a collimator. The beam splitter is oriented at an angle of 45.degree. to the direction of the incoming collimated beam so that a portion (i.e. percentage) of the impinging beam is transmitted without change of direction and a portion is reflected through a 90.degree. angle. Each portion is reflected back towards the beam splitter by a beam reversing mirror, the return optical path being coincident with the forward path. At the beam splitter, recombination of the two portions takes place with the first now progressing in reflection and the second in transmission towards a common path leading to a light receiving device such as a photoelectric detector.
If a monochromatic source is used, and if the movable mirror referred to earlier is actually displaced a predetermined limit by mounting the mirror on an accurate scanning assembly following a strictly rectilinear path, the output of the photodetector will be a sine wave of much lower frequency compared with the monochromatic emission line of the source but with a constant peak-to-peak amplitude. If the Fourier Transform is computed, the resulting trace, in the form of a very narrow band, represents the emission line spectrum of the source.
If a broad-band infrared source is substituted for the monochromatic infrared source, the output of the detector when the scanning mirror is in motion will no longer be a pure sine wave since the spectrum of the source will include waves of different frequencies. Each of the optical sine waves will give rise to two constituent beams and one resultant beam of different frequencies will be represented in the instantaneous output of the detector, each by its own modulation sine wave of related frequency and amplitude. If the output of the detector is plotted as before, the trace that results represents the interferogram of the source. If a sample not opaque to infrared (IR) radiation is placed in the beam path, the waves of different frequencies present in the spectrum of the source are attenuated to a different extent in a manner that is characteristic of the chemical nature of the sample and the resulting interferogram represents the infrared absorption of the sample superimposed on the interferogram of the source.
By taking the Fourier Transforms of the two interferograms, thus obtaining independently the spectrum of the sample-cum-source and that of the source alone, and then determining the ratio of the first spectrum and the second, the spectrum of the sample is derived.
In typical spectroscopy instruments, a sample is held in position in a focused infrared beam by a holding fixture which, as described in U.S. Pat. No. 4,695,727, typically has a housing with walls defining a sample holder. Each of the end walls has an opening, an inlet opening on one wall and an outlet opening on the other wall, with the sample holder being disposed between the walls and positioned in the beam path so that the beam passes between the inlet and outlet openings and through the sample holder. The sample holder itself consists of two thin holding elements, often referred to as windows, which are joined together to hold the sample in place. One obvious requirement of the material for the window is that it be transparent to infrared (IR) radiation. Such transparent windows are pervious to radiation. Materials satisfying this requirement are mostly inorganic salts such as NaCl, KBr, CaF.sub.2 and ZnSe. However, IR sample cells using these materials are too expensive to be disposable. An inexpensive disposable IR sample holder is marketed by 3M and is in the general form of a slide projector slide and is generally as described in U.S. Pat. No. 4,695,727. There is only one window. The window material is a porous polyethylene film. The sample to be measured is spread onto the window and is held in the pores of the polyethylene film. Because polyethylene has a unique IR spectrum, the absorption spectrum of the IR card typically interferes with the analysis of the sample. Although in principle the IR spectrum of polyethylene can be subtracted out, distortion of the spectrum usually still persists even in sophisticated interferometric spectrophotometers.
Therefore, what is needed is a new sample holder which retains a solid, semi-solid, gel or liquid sample without causing significant distortion of the spectrum.